Advances in electronics manufacturing technology have enabled the manufacture of inexpensive wireless devices in large quantities. Presently, there is an interest in greatly expanding the use of wireless networks beyond traditional communication uses.
It is expected that in the future wireless networks will be used for a variety of sensing, control, security, and location based services. The design of devices and protocols for operation of networks for such applications present numerous challenges. It is expected that in many such networks it will be necessary or desirable to have devices that can operate for extended periods of time on battery power. In the interest of reducing the cost of devices for such networks so that they can be widely deployed, it is believed that high accuracy, high cost crystal oscillators traditionally used in wireless communication devices may have to be dispensed with in favor of lower cost albeit lower accuracy components. Limiting the operating power reduces the amount of signaling and signal processing that can be used in maintaining and using a network. Using lower accuracy oscillators limits the ability of devices in a wireless network to stay in synchronization. Operating with low power tends to dictate infrequent signaling and reception, whereas use of low accuracy oscillators (which perforce leads to inter-device timing errors) makes it difficult to achieve coincidence of one device's infrequent signaling with another device's infrequent reception.
To address the foregoing difficulties co-pending patent application Ser. No. 09/803,322, entitled “A Multiple Access Protocol and Structure for Communication Devices in an Asynchronous Network” which is assigned in common with the instant application proposes a protocol according to which a number of devices in a network are able to operate with very low duty cycles. In low duty cycle operation, devices actively receive and transmit for relatively short periods compared to longer periods in which the devices are in a low powered sleep mode. Operating according to a low duty cycle leads to reduced power consumption, which as stated above is desirable or required for certain applications. Although such devices may have low accuracy oscillators, they are able to communicate through a number of devices that act as mediation devices. The mediation devices operate at a higher duty cycle than the other devices, and have relatively long receive periods during which they are able to receive communication from the low duty cycle devices. A low duty cycle device wanting to send a message (e.g., to another low duty cycle device) sends a ‘communication request’ message to a mediation device. Low duty cycle devices also periodically send ‘query’ messages to the mediation device to ascertain if they are the intended recipients of communications from other low duty cycle devices. The mediation devices receive and store the communication request messages, and upon receiving a query message from an intended recipient indicated in a communication request message, send a ‘response’ message to the querying device. The response message includes timing information for the device that initiated communication to the querying device by sending the communication request. The timing information allows the querying device to adjust its own timing to synchronize with the device that initiated communication, and thereafter directly (without going through the mediation device) communicate with the device that initiated communication.
Unfortunately, devices operating as mediation devices tend to consume more power. However, co-pending patent application Ser. No. 09/803,322 also discloses protocols in which devices in a network share the task of operating as mediation devices. In the latter case, the extra power consumption burden associated with operating as a mediation device will be shared among devices in a network.
Co-pending patent application Ser. No. 10/443,428 entitled “Media Access Control and Distributed Data Processing Using Mediation Devices in an Asynchronous Network” discloses protocols in which the aforementioned mediation devices take on additional functions including storing and forwarding data.
The protocols described in the abovementioned co-pending applications are suitable for so-called “ad-hoc” networks. In forming ad-hoc networks, a number of devices are placed in a building or outdoors so that subsets of the devices are within radio communication range of each other. The devices then interoperate to form a connected network. The placement of the devices may be dictated by application dependent needs, (e.g., a device may need to be placed at each smoke detector in a fire alarm application) and is not necessarily ideal from a network design point of view. The topology of such ad-hoc networks, which is in part dictated by the network formation protocol that is used and in part by the locations of the devices (dictated by application, and beyond the control of the protocol designer) can, in the case of protocols using mediation devices, lead to some of the mediation devices handling a greater communication load burden. Devices handling a greater communication load burden may, in the case of battery powered operation, exhaust their batteries first, and impair or disable the network, even if the batteries of most devices still have a substantial amount of energy remaining. Furthermore forcing a particular mediation device to bear a greater communication load could, for certain protocols and for heavy network traffic conditions, lead to increased message latency.
What is needed is a protocol for ad-hoc networks using mediation devices that facilitates sharing the burden of communication tasks that expend power and/or communication bandwidth.